Modern printers use a variety of inks to generate images from data. These inks may include liquid ink, dry ink, also known as toner, and solid ink. So-called “solid ink” refers to ink that is loaded into a printer as a solid, which is typically in stick or pellet form. The solid ink is melted within the printer to produce liquid ink that is supplied to a print head for ejection onto media or an intermediate member to generate a printed image from image data. These solid ink printers typically provide more vibrant color images than toner or liquid ink jet printers.
A schematic diagram for a typical solid ink imaging device is illustrated in FIG. 1. The solid ink imaging device, hereafter simply referred to as a printer 108, has an ink loader 110 that receives and stages solid ink sticks. The ink sticks progress through a feed channel of the loader 110 until they reach an ink melt unit 120. The ink melt unit 120 heats the portion of an ink stick impinging on the ink melt unit 120 to a temperature at which the ink stick melts. The liquefied ink is supplied to one or more print heads 130 by gravity, pump action, or both. Printer controller 180 uses the image data to be reproduced to control the print heads 130 and eject ink onto a rotating print drum or image receiving member 140 as image pixels for a printed image. Media 170, such as paper or other recording substrates, are fed from a sheet feeder 160 to a position where the image on the drum 140 can be transferred to the media. To facilitate the image transfer process, the media 170 are fed into a nip between the transfer, sometimes called transfix, roller 150 and the rotating print drum 140. In the nip, the transfix roller 150 presses the media 170 against the print drum 140. Offset printing refers to a process, such as the one just described, of generating an ink or toner image on an intermediate member and then transferring the image onto some recording media or another member.
Generation of images on the print drum may require several revolutions of the drum. In order to eject the ink in the proper position within a partially formed image, the precise position of the drum must be monitored. Additionally, the controller synchronizing the finished image with the feeding of a media sheet into a nip with the print drum for the transfer of the image from the drum to the media sheet needs accurate information regarding the position of the drum's surface as it rotates about its center. The printer 100, therefore, includes a rotary encoder that generates an electrical signal corresponding to the angular position of the rotating drum 140.
Rotary encoders may use optical, magnetic, or inductive sensing to generate a position signal. Optical encoders include a light source and sensor that are mounted on opposite sides of a flat disk. The disk is coupled to the rotating shaft of a print drum so the disk rotates with the shaft and drum. A plurality of spaced apart marks is located within a circumferential slot on the disk and this slot is positioned between the light source and light sensor. As the disk rotates, the light from the light source is interrupted by the marks. Consequently, the light sensor detects light in an on/off pattern corresponding to the marks on the disk as they pass between the light and its sensor. The resulting optical digital signal is converted by the sensor into an electrical digital signal. This signal may be used by a controller in a known manner for coordinating control of the print heads with an image on the print drum and for transferring an image from the print drum to a sheet of media. The disk bearing the series of spaced apart marks is sometimes known as a code wheel.
Optical encoders fall into two broad categories. The first category includes encoders that are assembled with a shaft extending from the center of the code wheel through the body or housing of the encoder. This type of encoder is delivered as a complete package for attachment via a coupler to the shaft about which the print drum rotates. The assembly of the encoder at the manufacturer's facility enables the code wheel, optical sensor, and light source to aligned and spaced from one another at the factory. The second category of encoders, sometimes referred to as modular encoders, do not have a shaft section built into the body or housing of the encoder. Instead, the code wheel typically has an annular opening at its center and the center of the housing so a collar to which the code wheel is mounted can be coupled to the shaft of the motor. The coupling may be accomplished using a set screw or the like. Various structures have been developed for axial and radial alignment of the code wheel so the code wheel is centered on the shaft and appropriate tolerances are provided for the placement of the code wheel within the gap between the light source and sensor of the encoder. These structures and tools require time during the installation of the encoder on the motor shaft for the alignment of the encoder components that are critical to accurate signal generation. Should the motor later require maintenance, the encoder must be removed and the alignment procedure repeated before returning the motor to service.